It is known that the graphite epoxy structural components used on aircraft are subject to natural lightning strike discharges. The most severe strikes are directed toward small-radius structures located at or near extending surfaces of the aircraft (wing tips, stabilizer tips, vertical tips, rudder, elevators, ailerons, engine cowlings, etc.). These locations are designated as "Zone 1" and are subjected to the initial attachment of the lightning channel. The initial attachment lightning stroke is characterized by a fast-rise, high-peak current (2.times.10.sup.5 amps) and a large energy transfer (2.times.10.sup.6 amps.sup.2 sec). It can create severe structural damage to unprotected graphite epoxy structures (much more damage than to comparable aluminum structures).
Other parts of the structure will be subjected to lesser discharges, called "swept stroke lightning." These areas are designated as "Zone 2" and are located aft of the original attachment points. Swept stroke lightning also contains a fast-rise current, but with a 1.times.10.sup.5 amp peak and an energy transfer of 0.25.times.10.sup.6 amps.sup.2 sec.
Particularly sensitive areas are integral fuel tanks and pressurized sections. Punctures cannot be tolerated in either area, but fasteners penetrating the skin and protruding into a fuel tank area have been shown to constitute a fuel ignition source even without a skin puncture. Unprotected fasteners are a preferred attachment point for the lightning and spark because the energy cannot be distributed fast enough into the surrounding graphite epoxy material (due to its low thermal and electrical conductivity).
Representative of the use of lightning protection systems in the prior art literature are U.S. Pat. Nos. 3,755,713; 4,479,163; and 4,502,092, assigned to The Boeing Company. Knitted wire mesh is utilized in the lightning protection system of U.S. Pat. No. 3,755,713, in contrast to the preferred embodiments of U.S. Pat. Nos. 4,479,163 and 4,502,092, which utilize nickel-plated graphite fibers. U.S. Pat. No. 3,755,713 shows only a decorative layer of knitted wire mesh, this layer providing insufficient coverage for any fastener heads. Further, this patent discloses no insulation between the fasteners and the structure since the reference structure is Fiberglas and, therefore, nonconductive.
U.S. Pat. No. 4,479,163 discloses an integral lightning protection system for composite aircraft skins. In this system, the graphite epoxy skin is covered by a nickel-plated graphite epoxy fabric comprising individual nickel-plated graphite fibers which are woven into the outer layer of fabric on the structural components requiring protection. Depending upon the protection desired (for Zone 1 or Zone 2), a varying percentage of the fiber tows in both the warp and fill direction of the fabric can be plated (for example, every second tow or every third tow).
When fasteners pierce through the skin, in order, for example, to attach internal structural members, such as L-beams, they are placed in holes through the skin, and reach into the interior of the compartment confined by the skin. The head of the fastener is at the same outer level as the graphite epoxy fabric ply. Thereafter, a coating of primer and paint can be applied to the fiber ply.
In one embodiment of U.S. Pat. No. 4,479,163, a fastener passes through the skin from the outside to the inside and the barrel of the hole through the skin is electrically insulated from the fastener by a cylindrical Fiberglas insulator. The graphite epoxy fiber fabric is then overlaid by a dielectric plastic strip, and the plastic strip is covered by a coating of primer and paint.
In U.S. Pat. No. 4,502,092, counterbore fasteners pass through the skin from outside to inside, where they are attached to structural elements. In various embodiments, the structural elements can be electrically insulated from the fastener, the structural elements themselves can be made from nonconductive material, and/or the internal panels, which are fastened in the interior of the compartment defined by the skin, can be electrically insulated from the structural support members attached to the skin. In each of the various embodiments of this patent, however, the fasteners are countersunk into the skin by forming a hole through the already-existing layer of graphite epoxy fiber fabric, inserting the fastener and filling the remaining hole with potting compound. A dielectric layer can be attached over the graphite epoxy fiber fabric and potting compound layer, followed by a coating of primer and paint.
For a variety of reasons, such as accidental damage or a later decision to add an internal component, it may be necessary to repair a graphite epoxy fiber fabric ply. The fabric ply need not have necessarily been either primed and painted or covered with one or more plies of metal-plated graphite epoxy fibers. It has been found in practice that following such a repair, the area of the repair is particularly susceptible to lightning strikes, even if the graphite epoxy structure is covered by at least one layer of metal-plated graphite fiber fabric. The reason is that state-of-the-art adhesives are nonconductive and, therefore, inhibit the dispersion of lightning currents out of the repair area. In this area of concentrated current, when the dielectric adhesive used to make the repair breaks down, internal and external arcing results, presenting a fuel ignition hazard and producing structural damage. It has also been found that for high-energy strikes, additional protection is required for fasteners penetrating the graphite epoxy skin already covered by a nickel-plated graphite fabric layer.